The mechanism of cell surface recognition and cell-contact mediated signal transduction will be studied in the biflagellated alga, Chlamydomonas reinhardtii. During the mating reaction, gametes of opposite mating types adhere to each other via extrinsic cell adhesion molecules, agglutinins, on their flagella and ultimately fuse to form zygotes. The binding of complementary flagellar agglutinins induces a 10-fold increase in intracellular levels of cAMP within minutes after gametes are mixed together. This second messenger generates several cellular responses most or all of which can be induced by addition of dibutyryl cAMP to gametes of a single mating type. These responses include: collapse of a functional barrier between the cell body and flagella, permitting migration and activation of a preexisting pool of agglutinins from the cell body membrane to the flagellar membrane; regulated secretion of a newly discovered serine protease into the periplasm, thereby activating a metalloprotease that releases the cell wall; and polymerization of actin filaments within a newly forming 4(mu)m-long acrosome-like extension called the fertilization tubule that initiates cell fusion. Within minutes after gametes have fused to form zygotes, their flagellar agglutinins become inactive in situ (although they can still be detected by an in vitro assay) and ultimately are shed into the medium. The research proposed in this application is intended to accomplish the following: 1) Determination of the molecular mechanisms underlying the signalled migration of agglutinins from the cell body plasma membrane to the flagellar membrane as well as identification of the mechanism of activation of the agglutinin molecules that accompanies migration. 2) Identification and characterization of the protein that anchors the agglutinins in the plasma membrane. 3) Determination of the mechanism of signal transduction by identifying and characterizing the flagellar adenyly cyclase and studying its interaction with the agglutinin/anchor protein complex. 4) Initiation of molecular genetic studies on agglutinin and signal transduction.